The present invention relates to a method for imparting releasability to the surface of a substrate such as a plastic film or paper by use of a curable organopolysiloxane composition or, more preferably, to a method for the preparation of a release paper having a surface film of a cured organopolysiloxane composition, of which the curing reaction of the composition can be completed at a relatively low temperature and the surface film of the cured organopolysiloxane composition is stable over a length of time and free from the problem of transfer to the adhesive surface to which the release paper is applied and bonded.
As is well known, the surfaces of sticky materials such as pressure-sensitive adhesive tapes coated with a sticking adhesive are temporarily protected from inadvertent sticking by applying and bonding a release paper thereto, which is a paper provided with a surface film having releasability by coating with a releasing composition. Among various types of releasing compositions, the most widely used ones are the oreganopolysiloxane-based compositions comprising an organopolysiloxane having two or more of aliphatically unsaturated hydrocarbon groups bonded to the silicon atoms in a molecule, an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms and a platinum catalyst with optional admixture of a stabilizer and solvent, in which the crosslinking reaction proceeds by the addition reaction between the silicon-bonded unsaturated hydrocarbon groups and the silicon-bonded hydrogen atoms.
The organohydrogenpolysiloxane conventionally used in the above mentioned surface-releasing organopolysiloxane composition of the addition reaction type is a methylhydrogenpolysiloxane having a substantially linear molecular structure and represented by the general formula EQU Me.sub.3 Si--O--SiHMe--O).sub.p (SiMe.sub.2 --O).sub.q SiMe.sub.3,
in which the Me denotes a methyl group and the subscripts p and q are each a positive integer. Of the two types of the siloxane units, i.e. methylhydrogensiloxane units and dimethylsiloxane units, in the methylhydrogenpolysiloxane, the dimethylsiloxane units form only a minor portion of the overall siloxane units. Accordingly, a considerably large number of the silicon-bonded hydrogen atoms remain unreacted even when the curing reaction of the composition has been seemingly complete and the residual silicon-bonded hydrogen atoms pertain to the reaction with the silicon-bonded unsaturated hydrocarbon groups in the lapse of time to gradually increase the density of crosslinks so that the releasability of the release paper is subject to change in time. When curing of the organopolysiloxane composition is performed by heating at a relatively low temperature or by ultraviolet irradiation for a short time, a small portion of the organopolysiloxane remains uncrosslinked and may be sometimes transferred to the sticking adhesive surface or to the back surface of paper in a roll or stack so as to cause decrease in the stickiness of the sticking adhesive or repellency of inks and paints on the surface.
Since it is a usual formulation of the organopolysiloxane-based releasing composition in order to achieve full curing that the organohydrogenpolysiloxane is used in such an excess amount that the silicon-bonded hydrogen atoms are provided in an amount of 1.5 moles or larger per mole of the silicon-bonded unsaturated hydrocarbon groups in the organopolysiloxane, it is naturally unavoidable that a large number of the silicon-bonded hydrogen atoms are left unreacted even when the addition reaction with the silicon-bonded unsaturated groups has proceeded to an extent of very completion. Such a residual amount of the silicon-bonded hydrogen atoms is of course responsible for the changes in time caused in the interface with the adhesive surface so that the peeling resistance of the release paper from the adhesive surface is gradually increased in the lapse of time.
Various attempts and proposals have been made hitherto to improve the organopolysiloxane-based releasing composition in this regard but the improvements hitherto undertaken are mainly in connection with the platinum catalyst, by which no fundamental solution of the above mentioned problems has yet been obtained although the curing velocity of the composition can seemingly be increased.